Process for producing acrylonitrile polymers

ABSTRACT

Acrylonitrile polymers of improved whiteness and thermal stability are provided by polymerizing acrylonitrile alone or with a comonomer in an aqueous medium, in the presence of a water-insoluble mercaptan, together with a redox catalyst comprising sulfurous acid or a salt thereof and nitrous acid or a salt thereof.

United States Patent Tamura 5] Oct. 28, 1975 PROCESS FOR PRODUCING2,676,952 4/1954 Ham 260/85.5 R ACRYLONITRILE POLYMERS 2,740,773 4/1956Ham 260/85.5 N 3,025,278 3/1962 260/85.5 R Inventor: filtoshl Tamura,Otake, Japan 3,028,372 4/1962 Kocay et al. 260/85.5 R 3,123,588 3/1964Lunney 260/85.5 R [73] 3,126,361 3/1964 Glabisch et al. 260/85.5 R3,213,069 10/1965 Rausch 260/85.5 N [22] Filed: Apr. 15, 1974 3,681,3118/1972 Patron et al. 260/85.5 R

21 Appl. No.2 460,678

Przmary Examiner-Harry Wong, Jr. Attorney, Agent, or Firm-Oblon, Fisher,Spivak [30] Foreign Application Priority Data Mcclelland & M

May 1, 1973 Japan 48-49397 [52] US. Cl.. 260/85.5 R; 260/78.5 R;260/79.3 M; [57] ABSTRACT 260/88.7 D; 260/88.7 F [51] Int. Cl 2 C08F3/76 C081; 15,02, CO8F 15/22 Acrylomtnle polymers of lmproved whitenessand [58] Field of Search zo/ss 5 R D 85 5 N thermal stability areprovided by polymerizing acrylo- 26O/88 7 7 7 nitrile alone or with acomonomer in an aqueous medium, in the presence of a water-insolublemercaptan, together with a redox catalyst comprising sulfurous [56]References cued acid or a salt thereof and nitrous acid or a saltthereof.

UNITED STATES PATENTS 7/1952 Conard et al. 260/85.5 N

2 Claims, N0 Drawings PROCESS FOR PRODUCING ACRYLONITRILE POLYMERSBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a process of polymerizing or copolymerizing acrylonitrile inan aqueous medium in the presence of a redox catalyst to produce anacrylonitrile homopolymer or copolymer of improved whiteness and thermalstability.

A catalyst combination of an oxidizing agent and a reducing agent, i.e.,a redox catalyst, is widely used for polymerizing in an aqueous mediumacrylonitrile or copolymerizing acrylonitrile with anothercopolymerizable monoethylenically unsaturated compounds. However, theacrylonitrile polymers and copolymers pre pared by the polymerization inan aqueous medium in the presence of a redox catalyst are not alwayssatisfactory because they are frequently characterized by poor whitenessand poor thermal stability.

It has now been found that when the polymerization or copolymerizationof acrylonitrile is performed in the presence of a specified redoxcatalyst, which comprises sulfurous acid, or its salts and nitrous acidor its salts, and of a water-insoluble mercaptan, polymers or copolymersof acrylonitrile of improved whiteness and thermal stability areobtainable with advantage.

2. Description of Prior Art In general, mercaptans act as chain transferagents in the polymerization, and therefore are widely used as molecularweight controlling agents to produce a polymer of desirably lowermolecular weight. It is already known, as disclosed in Japanese PatentPublication No. 1619/1968, that the addition of a water-solublemercaptan to the polymerization system involving a particular redoxcatalyst will yield a polymer of improved whiteness and thermalstability.

However, the addition of a water-soluble mercaptan is not advantageous,because this inevitably reduces conversion and increases the degree ofpolymerization as substantiated in the Comparative Examples below, andreferred to in the Japanese Patent Publication No. 1619/1968. Thereduction in conversion leads to a reduction in productivity ofpolymers, and an increase in the cost of production. In order to controlthe increase in the degree of polymerization, it is necessary to use agreater amount of the catalyst. Further, the reduction of the conversionand the increase in the degree of polymerization make it difficult toensure a stable operation. If a water-soluble mercaptan is employed in afar less amount, in order to minimize the disadvantages mentioned above,polymers of improved whiteness and thermal stability are not obtainable.

SUMlvIARY OF THE INVENTION Accordingly, it is an object of the presentinvention to provide a process which enables the production ofacrylonitrile polymers or copolymers having improved whiteness andthermal stability without the disadvantages mentioned above.

In accordance with the present invention there is provided a process forthe production of acrylonitrile polymers or copolymers whereinacrylonitrile or a mixture comprising at least 85% by weight ofacrylonitrile and at most 15% by weight of another copolymerizablemonoethylenically unsaturated compound is polymerized in an aqueousmedium, in the presence of a redox catalyst, comprising nitrous acid ora salt thereof, a sulfurous acid or a salt thereof, characterized inthat said polymerization is performed in the presence of awaterinsoluble mercaptan.

The monomer to be polymerized by the process of this invention isacrylonitrile or a mixture of acrylonitrile and another comonomer. Themixture may contain at most 15% by weight of a monoethylenicallyunsaturated compound copolymerizable with acrylonitrile. Suitablemonoethylenically unsaturated compound include, for example, vinylcompounds, such as vinyl acetate, vinyl chloride, vinylidene choride,acrylic acid and its esters, styrene, N-vinylsuccinimide, acrylamide,and vinylpyridine; and methacrylic acid and its esters, crotonic acid,and maleic anhydride.

The sulfurous acid or its salts, which constitute the other component ofthe redox catalyst used in the process of the present invention, includefor example, sulfurous acid, sulfites such as sodium sulfite, potassiumsulfite and ammonium sulfite; bisulfites such as sodium bisulfite,potassium bisulfite and ammonium bisulfite; and metabisulfites such assodium metabisulfite and potassium metabisulfite. These sulfurous acidand salts are hereinafter referred to as sulfurous compound for brevity.

The nitrous acid or its salts, which constitute one component of theredox catalyst used in the process of the present invention, include forexample nitrous acid and nitrites such as sodium nitrite, potassiumnitrite and ammonium nitrite. These nitrous acid and their salts arehereinafter referred to as nitrous compound for brevity.

Hydroxylamine sulfonates or their salts, are prepared easily fromnitrous acid or its salts and sulfurous acid or its salts, may be usedas the nitrous compound component of the redox catalyst. By saltsthereof" is intended the soluble salts, such as the alkali or ammoniumsalts.

The ratio of the nitrous compound to the sulfurous compound ispreferably 1/30 to A: by weight, and more preferably l/10 to US byweight. The sulfurous compound, should be present in an amount ofpreferably 1 [10,000 to l/50 by weight and more preferably 1/300 to 1/by weight based on the amount of water in the aqeuous polymerizationsystem. However, these amounts, of the nitrous compound and thesulfurous compound are not critical, because the amount of apolymerization initiator may be suitably determined taking intoconsideration the polymerization apparatus employed, the characteristicsof the resulting polymer, and costs.

The water insoluble mercaptans used in the process of the presentinvention, include alkylmercaptans, the alkyl group having at least 4carbon atoms, such as nbutyl mercaptan, t-butyl mercaptan, t-dodecylmercaptan and n-cetyl mercaptan; and esters of mercaptan carboxylicacids such as isopropyl thioglycolate and dodecyl thioglycolate.

The suitable amount of the water-insoluble mercaptans can vary dependingupon the particular mercaptan employed. The presence of only a verysmall amount of mercaptan in the polymerization system produces somebeneficial results. However, generally at least 0.005% by weight basedon the weight of monomer should be employed in order to produce polymersof a relatively high degree of whiteness and thermal stability. Whilethere is apparently no critical upper limit with regard difficult toobtain a desired degree of polymerization, even if the amount ofcatalyst used is suitably varied.

The manner of addition of the mercaptan to the polymerization system isnot critical. However, it is preferable to add the mercaptan in the formof solution, in the monomer which is to be polymerized, since themercaptan is insoluble in water but is readily soluble in the monomer tobe polymerized. It is believed that the presence of the mercaptanexercises the function of improving whiteness and thermal stability inthe form of being dissolved in the monomer.

The polymerization or copolymerization of the present invention isusually performed at a temperature of 30 to 70C and a pH of 1.5 to 5.0.

The monomer/water ratio fed into the polymerization system shouldpreferably to 1 to 55 by weight and more preferably l/7 to l/S. When theproportion excessively increases, it becomes difficult to remove theheat of polymerization and to prevent the polymer particles fromdepositing on the inner wall of the polymerization reactor and touniformly agitate the polymerization mixture, although the amount of thecatalyst used is reduced and the conversion and productivity areincreased. Accordingly, the conditions, under which the polymerizationor copolymerization is performed, should be determined based on thedesired degree of polymerization or other physical properties of thepolymer.

In the polymerization or copolymerization, suspension stabilizers,emulsifiers and coagulating agents may be employed, if required. Thepolymerization or copolymerization may be performed in either acontinuous or batchwise manner.

The reason whiteness and thermal stability of the polymer areenhancedwith only a slight reduction in the conversion and reduction in thedegree of polymerization is assumed to be 'as follows:

'In general, it is accepted that when acrylonitrile is poof end groupsstabilized with the mercaptans. In this instance, chain transfer fromthe catalyst to the mercaptans will inevitably occur because thecatalyst is dissolved in water and, therefore, the catalyst radicalseffective for the polymerization are undesirably decreased, which leadsto a reduction in conversion and an increase in the degree ofpolymerization.

- In contrast, when the mercaptans employed are water-insoluble, almostthe total amount of the mercaptans will be present in the monomersadsorbed on the polymer particles and, hence, will contribute to theformation of end groups stabilized with the mercaptans. Chain transferfrom the catalyst to the mercaptans does not substantially occur and,therefore, both the reduction in conversion and the increase in thedegree of polymerization can be controlled.

Further, it is to be noted that water-insoluble mercaptans are onlyeffective when they are employed together with the redox catalystcomprising the sulfurous compound and the nitrous compound.Water-insoluble mercaptans have little or no effectiveness when they areused together with typical redox catalysts such as those comprisingpersulfates and sulfites, as substantiated in Reference Example 2described hereinafter. The reason for this is assumed to be that whensuch redox catalysts of enhanced activity are employed, the.

polymerization rate is too fast to permit the adsorption of monomer ontothe growing polymer particles.

The water-insoluble mercaptans employed are not completely separatedfrom the polymer in the step of filtration or washing after thecompletion of polymerization. Accordingly, a large amount of thewaterinsoluble mercaptans will remain in the finished polymer product.However, such water'insoluble mercaptans do not detrimentally influencethe polymer. They may exert some advantageous effects on the polymer,for example, enhance the thermal stability of the polymer, although onlyto a slight extent, as seen in Reference Example 1 describedhereinafter. It is to be noted, however, that advantageous effectsproduced by the process of the present invention are mainly due to thefact that water-insoluble mercaptans are present in the polymerizationsystem. i

The invention is further illustrated by reference to the followingexamples in which parts and percentages are by weight unless otherwiseindicated.

In the following examples, the reduced viscosity was measured asfollows:

Principle Reduced viscosity of polymer is determined by measuring theefilux time of a 0.2% polymer solution in dimethyl formamide (DMF) at259C using at Ostwald- Cannon-Fenski viscosimeter. The efflux time ofthe solvent is measured similarly, and the reduced viscosity can then becalculated.

EXANIPLE 1 Into a polymerization reactor, were charged first, 700 partsof water and, then, 0.6 parts of sodium nitrite, 6 parts ofmetabisulfite and 0.5 part of sodium methallylsulfonate. After thesewere dissolved, sulfuric acid was added to the solution to adjust the pHof the solution to 2.5. Then, a monomer mixture of 94 parts ofacrylonitrile and 6 parts of methyl acrylate was added to the solutionat a constant rate over a period of minutes. The polymerization wasperformed at a temperature of 55C under an atmosphere of air for 4 hourswhile the solution was stirred.

In Experiment No. 1, the polymerization was performed in the absence ofmercaptan. In Experiments No. 2 and 3, the polymerization was performedin the presence of 0. 1% and 0.2% of n-octyl mercaptan, based on theweight of the monomer mixture respectively. The n-octyl mercaptan wasadded in to the solution after being dissolved in a part of the monomermixture. In Experiments No. 4 and 5, the polymerization was performed inthe presence of 0. 1% and 0.2% of thioglycolic acid, based on the weightof the monomer mixture, respectively. The thioglycolic acid was added ina manner similar to that in the case of n-octyl mercaptan.

After completion of the polymerization, the polymer so obtained waswashed with a great amount of water and then dried. The polymer wasfound to have the degree of polymerization, the reduced viscosity andthe absorbance as shown in Table I. The reduced viscosity was determinedat 25C in a solution of a concentration of 0.2 g per 100 ccofdimethylformamide. The absorbance was determined by maintaining apolymer solution of a concentration in dimethylformamide at atemperature of 90C for 90 minutes, and then measuring the absorbance at430 111 microns and 460 m microns.

Note based on the weight of monomer As seen from Table I, noctylmercaptan, which is insoluble in water, results in a polymer of reducedcolor and improved whiteness, even when the amount of the mercaptan usedis 0.1%, as compared with a polymer prepared without the addition ofmercaptan. The reduction in conversion caused by the addition of n-octylmercaptan is negligibly small. The reduced viscosity of the polymerobtained by the addition of n-octyl mercaptan is almost similar to thatof the polymer prepared without the addition thereof when the amount ofaddition is 0.1%, and desirably lower when the amount of addition is0.2%.

In contrast, thioglycolic acid, which is soluble in water, results in apolymer of reduced color but increased reduced viscosity with a reducedconversion.

In orderto minimize or obviate such defects, tests were repeated whereinthe amounts of thioglycolic acid used were varied. However, it was foundto be impossible to reduce the whiteness without the reduction ofconversion.

until the amount of sulfurous acid dissolved therein reached 2 parts interms of S0 T0 the aqueous solution, were added, first, 0.6 parts ofsodium sulfite and, then sodium bicarbonate and sulfuric acid to adjustthe pH of the aqueous solution to 2.5. Then, a monomer mixture of 94parts of acrylonitrile and 6 parts of acrylic acid was added to theaqueous solution at a constant rate over a period of 80 minutes. Thepolymerization was performed at a temperature of C under an atmosphereof air for 4 hours while the solution was stirred.

In Experiment No. 6, the polymerization was performed in the absence ofmercaptan. In Experiments No. 7 and 8, 0.1% and 0.2% of n-dodecylmercaptan were used, respectively.

In Experiments No. 9 and 10, 0.05% and 0.1% of 2- mercapto-ethanol wereused, respectively. All these mercaptans were added in the form of asolution dissolved in monomer.

After completion of the polymerization, the polymer so obtained wasrecovered and then tested in the same manner as in Example 1. Resultsare shown in Table II.

As seen from Table II, n-dodecyl mercaptan which is insoluble in waterand 2-mercaptoethanol which is soluble in water have functions similarto those of n-octyl mercaptan and thioglycolic acid, respectively, bothof which were used in Example 1.

EXAMPLE 3 The procedure described in Example 1, Experiment No. 2 wasrepeated wherein various water-insoluble mercaptans were separately usedin substitution for noctyl mercaptan. Test results are shown in Table111.

EXAMPLE 2 700 parts of water was charged in a polymerization reactor.Sulfur dioxide gas was blown into the water As seen from Table III, theaddition of waterinsoluble mercaptans improves the whiteness andsuppresses the color formation without substantial influence to theconversion and the reduced viscosity.

EXAMPLE 4 Continuously added into a continuous polymerization reactorwere 91 parts ofacrylonitrile, 9 parts of vinyl acetate, 0.5 parts ofsodium methallylsulfonate, 550 parts of water, 1 part of sodiumhydroxylamine sulfonate, sodium metabisulfite and sulfuric acid. Theamount of sodium metabisulfite was such that polymers contained in thepolymerization mixture had a reduced viscosity of 1.65. The amount ofsulfuric acid was such that the pH of the polymerization mixture was2.5. The polymerization temperature was 55C and average residence timewas 8 hours. A polymer slurry continuously withdrawn from the reactorwas washed with a great amount of water, dehydrated, dried andpulverized.

In the procedure described above, water was charged into thepolymerization reactor in an amount of about twothirds in volume of thepolymerization reactor, followed by the addition of a part of the sodiumhydroxylamine sulfonate and the sodium metabisulfite and, then, thesulfuric acid was added to adjust the pH of the aqueous solution to 2.5,before the monomer feed was introduced into the polymerization reactor,to prepare for the initiation of polymerization.

In Experiment No. 18, 0.08%, based on the weight of monomer, of n-octylmercaptan was added in the form of a solution in the monomer mixture. Incontrast, in Experiment N0. 19, no mercaptan was employed. Tests As seenfrom Table IV, the addition of n-octyl mercaptan reduces the colorformation more than, and provides conversion and reduced viscositysubstantially equal to, that obtained without the addition of mercaptan.

Filaments were prepared by a conventional procedure from the polymermentioned above. The filaments from the polymer of Experiment No. 18were found to be superior in whiteness and thermal stability to thosefrom the polymer of Experiment No. 19.

The procedure of Example 4, Experiment No. 18, was repeated whereinsodium hydrogen sulfite was em ployed instead of sodium metabisulfite.Test results of the polymer was approximately the same as in Example 4,Experiment No. 18.

REFERENCE EXAh/[PLE 1 Solutions of n-octyl mercaptan indimethylformamide having various concentrations were prepared. Thepolymer prepared in Example 4, Experiment No. 19, was dissolved in eachof the n-octyl mercaptan solutions to form the polymer solution of aconcentration. Absorbance test was made on the 10% polymer solution.Results are shown in Table V. The results in Example 4, Experiment No.4, are also shown in Table V for comparison purposes.

Note "based on the weight of the solution in dimethylfonrlamide As seenfrom Table V, in the case where n-octyl mercaptan is added to a polymersolution, the resulting solution exhibits improved thermal stabilityonly to a slight extent as compared to the polymer solution containingno mercaptan. Accordingly, it may be concluded that water-insolublemercaptans effectively function only during the polymerization and areineffective when they are added after the polymerization.

REFERENCE EXAMPLE 2 240 parts of water was charged into a polymerizationreactor followed by the addition of 0.00005 part, in terms of iron, offerrous sulfate. Sulfuric acid was added to adjust the pH to 2.5. Thento the solution were added at a constant rate over a period of minutes,a monomer mixture of 89.5 parts of acrylonitrile and 10.5 parts of vinylacetate, an aqueous solution of 0.43 part of potassium persulfate and1.07 parts of sodium hydrogen carbonate in 70 parts of water, and 70parts of an aqueous sulfurous acid solution containing 0.98 part of S0prepared by blowing sulfur dioxide gas into water. The polymerizationwas performed at a temperature of 50C under an atmosphere of air for 1hour and 40 minutes while the solution was stirred.

In Experiment No. 23, no mercaptan was added. In Experiment No. 24,0.1%, based on the weight of monomer, of n-octyl mercaptan was added inthe form of a solution in monomer.

After completion of the polymerization, the polymer so obtained wasrecovered and tested in the same manner as that in Example 1. Resultsare shown in Table VI.

As seen from Table VI, the addition of n-octyl mercaptan to a redoxcatalyst system of potassium persulfate and sulfurous acid is notsubstantially effective for improving whiteness.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionas set forth herein.

What is claimed as new and intended to be secured by letters patent ofthe United States is:

1. A process for the production of acrylonitrile polymers or copolymerswherein acrylonitrile or a mixture comprising at least by weight ofacrylonitrile and at most 15% by weight of other copolymerizablemonoethylenically unsaturated compounds is polymerized in an aqueousmedium at 30C to 70C at a pH of 1.5 5.0, wherein the monomer to waterratio is l/10 to A1, in contact with a redox catalyst comprising nitrousacid or a salt thereof and sulfurous acid or a salt thereof, wherein theratio of said nitrous acid or salt to said sulfurous acid or saltthereof is 1/30 to as by weight, and the sulfurous acid or salt thereofis present in an amount of 1/10,000 to l/50 by weight, based on the ofsulfites, bisulfites and metabisulfites.

1. A PROCESS FOR THE PRODUCTION OF ACRYLONITRILE POLYMERS OR COPOLYMERSWHEREIN ACRYLONITRILE OR A MIXTURE COMPRISING AT LEAST 85% BY WEIGHT OFACRYLONITRILE AND AT MOST 15% BY WEIGHT OF OTHER COPOLYMERIZABLEMONOETHYLENICALLY UNSATURATED COMPOUNDS IS POLYMERIZED IN AN AQUEOUSMEDIUM AT 30*C TO 70*C AT A PH OF 1.5 -5.0, WHEREIN THE MONOMER TO WATERRATIO IS 1/10 TO 1/3, IN CONTACT WITH A REDOX CATALYST COMPRISINGNITROUS ACID OR A SALT THEREOF AND ULFUROUS ACID OR A SALT THEREOF,WHEREIN THE RATIO OF SAID NITROUS ACID OR SALT TO SAID SULFUROUS ACID ORSALT THEREOF IS1/30 TO 1/3 BY WEIGHT, AND THE SULFUROUS ACID OR SALTTHEREOF IS PRESENT IN AN AMOUNT OF 1/10,000 TO 1/50 BY WEIGHT, BASED ONTHE AMOUNT OF WATER IN THE AQUEOUS MEDIUM, AND FURTHER IN CONTACT WITH0.005% - 5% BY WEIGHT BASED ON THE WEIGHT OF MONOMER OF AWATER-INSOLUBLE MERCAPTANS TAN SELECTED FROM THE GROUP CONSISTING OFALKYL MERCAPTANS CONTAINING AT LEAST 4 CARBON ATOMS AND MERCAPTANCARBOXYLATE ACID ESTERS.
 2. The process according to claim 1 whereinsaid salt of sulfurous acid is selected from the group consisting ofsulfites, bisulfites and metabisulfites.